1. Field of the Invention
The present invention relates to a hologram, whose diffraction grating film has a large area, and which is excellent in water resistance.
2. Description of the Related Art
A hologram is a device for causing incident light for an image to be displayed to diffract and reflect so as to obtain a reproduction light, and enabling a viewer to recognize the image to be displayed owing to said reproduction light.
This hologram is used for a head up display device 9, as illustrated in e.g. FIG. 19.
The aforesaid head up display device 9 is provided with an indicator 90 having a light source, a hologram 91 for diffracting and reflecting a light 30 emitted from the aforesaid indicator 90, and a windshield 92 for causing a diffracted light 31 emitted from said hologram 91 to reflect so as to enable a viewer 93 to recognize an image 94 to be displayed. Onto the aforesaid windshield 92, there is deposited a deposition film 921 for reflecting the aforesaid diffracted light 31.
The hologram 91 consists, as illustrated in FIG. 20, of a glass substrate 912 with a diffraction grating film 97 provided thereon, and cover plates 913 and 914 sandwiching said glass substrate 912 therebetween. The cover glass 913 improves the image quality, for it is made of reflection free coat glass or the like. In addition, the cover glass 914 imparts water resistance to the hologram 91.
The glass substrate 912 is sealed between the cover glass plates 913 and 914 through an adhesive 910. As illustrated in FIG. 20 and FIG. 21, a sealing width S is provided on the outer circumferential portion 20 of a diffraction grating film 910, and the diffraction grating film 97 is tightly sealed with the adhesive 910.
In the preparation of the aforesaid hologram, as illustrated in FIG. 20 and FIG. 21, the diffraction grating film 97 with an interference fringe recorded thereon is formed on the glass substrate 912. Subsequently, the outer circumference 20 of the diffraction grating film 97 is eliminated as the sealing width S. Subsequently, the adhesive 910 is coated on the outer circumferential portion of the diffraction grating film 97 and that of the glass substrate 912, and the cover glass plate 914 is adhered to the adhesive 910. In addition, an adhesive 910 is coated also on a surface of the glass substrate 912, opposite to the diffraction grating film 97, and the cover plate 913 is adhered thereto. The hologram 91 is thereby obtained.
In the aforesaid conventional hologram, it is indispensable to provide a sealing width S ranging from 3 to 8 mm on the outer circumferential portion 20 of the diffraction grating film 97, in order to protect the diffraction grating film against ambient humidity. Owing to this sealing width, the area of the diffraction grating film 97 becomes markedly smaller than that of the hologram. The display area for the image 94 to be displayed is thereby restricted.
To reduce the sealing width, improved adhesives are necessary, which can be costly and time consuming to use.